The present invention relates to a hologram recording apparatus and method for multiplexed recording of a hologram on a hologram recording medium by an angle multiplex system.
In recent years, hologram recording/reproduction apparatuses for recording and reproducing a large capacity of data by utilizing the hologram technology have been proposed (see, for example, IBM J. RES DEVELOP VOL. 44 NO. 3 May 2000 “Holographic data storage”). In the hologram recording/reproduction apparatus, a technique called multiplex recording is used for enhancing the recording density. Unlike the conventional recording, this technique consists in recording a multiplicity of independent pages at a location. Representative examples of the multiplex recording system include angle multiplex recording, shift multiplex recording and phase code multiplex recording, and other various systems such as speckle multiplexing are also known.
FIG. 20 illustrates the case of recording on a hologram recording material (hologram recording medium) by the angle multiplex system. Laser light emitted from a laser light source (not shown) is branched into a signal beam and a reference beam, and the signal beam 100 having undergone spatial beam modulation in a beam spatial modulator (not shown) passes through a signal beam optical system (not shown) to be condensed on the hologram recording material 10. On the other hand, the reference beam 200 passes through a reference beam optical system (not shown) to be incident on a scan mirror 1, and is reflected by the scan mirror 1 to be incident on a lens 2 in the reference beam optical system. The lens 2 and a lens 3 constitute a 4f optical system, the reference beam 200 irradiates the hologram recording material 10 therewith through the lens 3, and an interference fringe generated by interference between the signal beam 100 and the reference beam 200 is recorded on the hologram recording material 10. Here, when the angle of the scan mirror 1 is varied, the angle of incidence of the reference beam 200 on the hologram recording material 10 is varied according to the angle variation, and holograms different on the basis of each incidence angle are multiplex recorded in the same recording region on the hologram recording material 10.
FIG. 21 illustrates the case where the area of irradiation of the hologram recording material 10 with the reference beam 200 is varied according to variation in the incidence angle of the reference beam 200. Since the diameter of the beam is constant irrespectively of a difference in the angle of incidence (with reference to the normal to the medium) of the reference beam 200 on the hologram recording material 10, the irradiation area in the case of incidence angle A is smaller than that in the case of incidence angle B, as is clear from the figure.
FIG. 22 illustrates the manner of a variation of the irradiation area in response to a variation in the incidence angle of the reference beam 200. Where the beam diameter of the reference beam 200 is φ1, as the angle of the scan mirror 1 becomes greater and the incidence angle of the reference beam 200 becomes greater as shown in FIGS. 10A, 10B and 10C, the diameter of the portion of irradiation of the hologram recording material 10 with the reference beam 200 becomes greater in the order of φ1, φ2 and φ3.
In the angle multiplex recording system, when the angle of incidence of the reference beam 200 on the hologram recording material 10 is varied, the signal beam 100 irradiates the same area portion of the hologram recording material 10 at a fixed incidence angle, irrespectively of the variation in the incidence angle of the reference beam 200. This means that, as the area of irradiation with the reference beam 200 is increased, the range of irradiation of the hologram recording material 10 with the reference beam 200 inclusive of the range of irradiation with the signal beam 100 is broadened. Intrinsically, however, it is desirable for the reference beam 200 to irradiate in such a range as to cover to a required minimum extent the range of irradiation of the hologram recording material 10 with the signal beam 100. This is because the irradiation of an unnecessary portion with the reference beam 200 would sensitize the hologram recording material 10 in excess accordingly, attended by wasting of the dynamic range thereof and leading to a lowering in the recording capacity.